Click chemistry is a flexible method featuring only the most feasible and efficient chemical reactions. The synthesis of 1,2,3-triazole from azides and terminal acetylenes using copper(I) as a catalyst is an extremely powerful reaction due to the extreme dependability, good selectivity, and biocompatibility of the starting materials. Triazole molecules are more than simple passive linkers; through hydrogen bonding and dipole interactions, they rapidly bind with biological targets. Its applications in drug development are expanding, ranging from target-oriented in situ chemistry and combinatorial mechanisms for lead generation to bioconjugation methods to study proteins and DNA. The click chemistry has frequently been used to speed up drug discovery and optimization processes in the past few years. The click chemistry reaction based on copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a biochemical process with applications in medicinal chemistry and chemical biology. Thus, click reactions are an essential component of the toolkit for medicinal chemistry and help medicinal chemists overcome the barriers in chemical reactions, increase throughput, and improve the standards of compound libraries. The review highlights the recent advancements in the copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry approach for synthesizing biologically important triazole moieties with a greater emphasis on synthesis methodologies and pharmacological applications. Additionally, the triazole-based FDA-approved drugs are also discussed with their mode of action to highlight the importance of the click chemistry approach in synthesizing the bioactive triazole compounds.

Recent advances in materials science and engineering highlight the importance of designing sophisticated biomaterials with well-defined architectures and tunable properties for emerging biomedical applications. Click chemistry, a powerful method allowing specific and controllable bioorthogonal reactions, has revolutionized our ability to make complex molecular structures with a high level of specificity, selectivity, and yield under mild conditions. These features combined with minimal byproduct formation have enabled the design of a wide range of macromolecular architectures from quick and versatile click reactions. Furthermore, copper-free click chemistry has resulted in a change of paradigm, allowing researchers to perform highly selective chemical reactions in biological environments to further understand the structure and function of cells. In living systems, introducing clickable groups into biomolecules such as polysaccharides (PSA) has been explored as a general approach to conduct medicinal chemistry and potentially help solve healthcare needs. De novo biosynthetic pathways for chemical synthesis have also been exploited and optimized to perform PSA-based bioconjugation inside living cells without interfering with their native processes or functions. This strategy obviates the need for laborious and costly chemical reactions which normally require extensive and time-consuming purification steps. Using these approaches, various PSA-based macromolecules have been manufactured as building blocks for the design of novel biomaterials. Clickable PSA provides a powerful and versatile toolbox for biomaterials scientists and will increasingly play a crucial role in the biomedical field. Specifically, bioclick reactions with PSA have been leveraged for the design of advanced drug delivery systems and minimally invasive injectable hydrogels. In this review article, we have outlined the key aspects and breadth of PSA-derived bioclick reactions as a powerful and versatile toolbox to design advanced polymeric biomaterials for biomedical applications such as molecular imaging, drug delivery, and tissue engineering. Additionally, we have also discussed the past achievements, present developments, and recent trends of clickable PSA-based biomaterials such as 3D printing, as well as their challenges, clinical translatability, and future perspectives.

Molecular hybridization is one of the recent stratagems in medicinal chemistry to synthesize a novel hybrid molecule having better affinity and efficacy by combining two or more pharmacophoric moieties. Molecular hybridization, i.e., a linker or framework integration technique, can be used to connect the two pharmacophoric components. It has often been found that hybrid compounds perform more effectively and possess lower toxicity than their parent molecules. In order to create a new generation of effective and safe therapeutic candidates, such as anti-cancer, anti-viral, anti-HIV, antioxidant, and antibacterial, for a variety of frontline diseases, several articles have been published that discuss the molecular hybridization of preclinically or clinically proven compounds. Isatin and its derivatives have been studied extensively due to diversified biological activities, including antitumor, antimicrobial, anti-inflammatory, analgesic, antiviral, antioxidant, anticonvulsant, etc. Similarly, 1,2,3-triazoles have received significant interest as a bio-isostere in medicinal chemistry for generating a large number of pharmaceutically significant molecules. As it possesses diversified physiochemical properties, such as hydrogen bond formation capacity, ease of synthesis, moderate dipole moment, stability towards acidic/basic hydrolysis, inertness towards oxidizing/ reducing agents, and good binding potential with several biological targets, triazole is an important choice of the medicinal chemists for the novel medication development. The aim of the current review is to summarize the research articles showing the pharmacological significance of hybrid molecules containing isatin and 1,2,3-triazole moieties. The present review may assist chemists in designing and synthesizing isatin-1,2,3-triazole hybrids with better efficacy and low cytotoxicity.

The privileged 1,2,3-triazole scaffold is drawing researcher\'s attention due to its widespread applications in diverse fields such as drug discovery (e.g., carboxyamidotriazole), organic synthesis (click-reaction template), polymeric materials (e.g., triazolamer), supramolecular receptors (e.g., triazolophane), fluorescent materials (e.g., metal-organic frameworks), and agricultural sectors (e.g., fungicides). Various 1,2,3-triazole persuasion modules are also currently available in the market that have multiple assets such as active pharmaceuticals and agricultural purposes. Owed to the highly consistent and firmest synthesis approach, that is, click reaction of various azides and acetylene derivatives by copper (I)-catalyzed 1,3-dipolar cycloaddition (CuAAC), highly functionalized 1,2,3-triazoles are prepared in scalar yields for drug discovery. Given the importance of 1,2,3-triazole chemistry, the present review focuses specifically on the synthesis of structurally diverse 1,2,3-triazoles linked to natural pharmacophores and their biological importance. Furthermore, the dual/multi-pharmacophores assimilated 1,2,3-triazoles have listed interesting biological activities that could be valuable as future drug leads. In addition, this comprehensive review can serve as a template for the development of new diverse scaffolds that will ensure for new therapeutic approaches for the existing myriad diseases and disorders.

The purpose of this review is to critically assess the kinetic behavior of the furan/maleimide Diels-Alder click reaction. The popularity of this reaction is evident and still continues to grow, which is likely attributed to its reversibility at temperatures above 100 °C, and due to its biobased \"roots\" in terms of raw materials. This chemistry is used to form thermoreversible crosslinks in polymer networks, and thus allows the polymer field to design strong, but also end-of-life recyclable thermosets and rubbers. In this context, the rate at which the forward reaction (Diels-Alder for crosslinking) and its reverse (retro Diels-Alder for decrosslinking) proceed as a function of temperature is of crucial importance in assessing the feasibility of the design in real-life products. Differences in kinetics based from various studies are not well understood, but are potentially caused by chemical side groups, mass transfer limitations, and the analysis methods being employed. In this work, all the relevant studies are attempted to be placed in perspective with respect to each other, and thereby offer a general guide is offered on how to assess their recycling kinetics.

Polysaccharides and proteins are important macromolecules for developing hydrogels devoted to biomedical applications. Chemical hydrogels offer chemical, mechanical, and dimensional stability than physical hydrogels due to the chemical bonds among the chains mediated by crosslinkers. There are many crosslinkers to synthesize polysaccharides and proteins based on hydrogels. In this review, we revisited the crosslinking reaction mechanisms between synthetic or natural crosslinkers and polysaccharides or proteins. The selected synthetic crosslinkers were glutaraldehyde, carbodiimide, boric acid, sodium trimetaphosphate, N,N\'-methylene bisacrylamide, and polycarboxylic acid, whereas the selected natural crosslinkers included transglutaminase, tyrosinase, horseradish peroxidase, laccase, sortase A, genipin, vanillin, tannic acid, and phytic acid. No less important are the reactions involving click chemistry and the macromolecular crosslinkers for polysaccharides and proteins. Literature examples of polysaccharides or proteins crosslinked by the different strategies were presented along with the corresponding highlights. The general mechanism involved in chemical crosslinking mediated by gamma and UV radiation was discussed, with particular attention to materials commonly used in digital light processing. The evaluation of crosslinking efficiency by gravimetric measurements, rheology, and spectroscopic techniques was presented. Finally, we presented the challenges and opportunities to create safe chemical hydrogels for biomedical applications.

Engineered block polysaccharides is a relatively new class of biomacromolecules consisting of chemical assembly of separate block structures at the chain termini. In contrast to conventional, laterally substituted polysaccharide derivatives, the block arrangement allows for much higher preservation of inherent chain properties such as biodegradability and stimuli-responsive self-assembly, while at the same time inducing new macromolecular properties. Abundant, carbon neutral, and even recalcitrant biomass is an excellent source of blocks, opening for numerous new uses of biomass for a wide range of novel biomaterials. Among a limited range of methodologies available for block conjugation, bifunctional linkers allowing for oxyamine and hydrazide \'click\' reactions have recently proven useful additions to the repertoire. This article focuses the chemistry and kinetics of these reactions. It also presents some new data with the aim to provide useful protocols and methods for general use towards new block polysaccharides.

BACKGROUND: Tailoring extracellular vesicles (EVs) can bequeath them with diverse functions and efficient performance in nano-biotechnology. Engineering and modification of EVs improves the targeted drug delivery efficiency. Here, we performed systematic review of various methods for EVs modifications.
METHODS: PubMed, Scopus, ISI Web of Science, EMBASE, and Google Scholar were searched for available articles on EVs modifications (up to March 2021). In total, 1208 articles were identified and assessed, and then only 36 articles were found eligible and included.
RESULTS: Six studies demonstrate the application of click chemistry, seven studies used co-incubation, two studies used chemical transfection, four studies implicated electroporation and sonication approach for modification of EVs. Moreover, two studies utilized microfluidics as suitable approach for loading cargo into EVs, while eight studies showed freeze-thaw method as feasible for these biological nanoparticles.
CONCLUSIONS: Freeze-thaw approach is found to be convenient and popular among researchers for performing modifications in EVs for the purpose of targeted drug delivery loading. Clinical-grade EVs production with good clinical practices (GCPs) is challenging in the current scenario. More studies are needed to determine the best suitable approach for cargo loading of EVs that may be exploited for research and therapeutic use.

Peptide-oligonucleotide conjugates (POCs) represent one of the increasingly successful albeit costly approaches to increasing the cellular uptake, tissue delivery, bioavailability, and, thus, overall efficiency of therapeutic nucleic acids, such as, antisense oligonucleotides and small interfering RNAs. This review puts the subject of chemical synthesis of POCs into the wider context of therapeutic oligonucleotides and the problem of nucleic acid drug delivery, cell-penetrating peptide structural types, the mechanisms of their intracellular transport, and the ways of application, which include the formation of non-covalent complexes with oligonucleotides (peptide additives) or covalent conjugation. The main strategies for the synthesis of POCs are viewed in detail, which are conceptually divided into (a) the stepwise solid-phase synthesis approach and (b) post-synthetic conjugation either in solution or on the solid phase, especially by means of various click chemistries. The relative advantages and disadvantages of both strategies are discussed and compared.

In this review, authors focus mostly on the various synthetic strategies developed so far for 1,2,3- triazole scaffold and its derivatives via different approaches such as metal-free, metal assisted or bimetallic. A brief overview of applications of the very important 1,2,3-triazole scaffold along with pharmacological activity is also discussed. Synthetic strategies are updated until June 2020.